Poly(ethylene glycol) or Poly(ethylene oxide)? Magnitude of end-group Contribution to the Partitioning of Ethylene Oxide Oligomers and Polymers between Water and Organic Phases

Ethylene oxide oligomers and polymers (hereafter referred to as PEO) are amongst the most commonly used substances in pharmaceutical and other industrial formulations. Depending on the oligomer/polymer molar mass, they have been called poly(ethylene glycol)s or poly(ethylene oxide)s, the former applied to smaller ones to suggest a glycolic chemical nature attributed to significant contributions from their hydroxyl end-groups. Just for guidance, catalogues from some common chemical companies place this transition between molar mass of 3350 or 10 000 g mol. More than a nomenclature issue, this end-group contribution may have important implications, as already stressed in relation to PEO solubility and miscibility in blends. In general, it is assumed that endgroup effects become less important, and, eventually, insignificant, as polymer molar mass increases, but no clear discrimination of this transition region for PEO has been presented yet. Most PEO applications depend on its high water solubility, a feature that has been related to a precise fitting of the ethylene oxide units into liquid water structure4 (for instance, poly(methylene oxide) and poly(propylene oxide) are considerably less soluble in water), in addition to favourable contributions from its hydroxyl end-groups. Despite this high aqueous solubility, there is a classical statement (found in Bailey ́s textbook) that PEO may be extracted from water to organic solvents, that has been ascribed to an entropy increase due to the loss of polymer chain helicity upon transfer from water to the apolar phase. Polymer partitioning is a key issue in the mechanism of formation of liquid biphasic systems (both aqueous and organic), as well as an important step in fractionation procedures. For PEO, it is of great importance when extended to biological systems, given its use, for instance, in cell fusion, where a key step has been proposed to be PEO interaction / partitioning at the biological membrane. In the last years, we have investigated PEO solution behaviour in both aqueous and organic biphasic systems, and shown that polymer solvation makes an important contribution to the driving forces controlling the formation of such systems. 10 Hence, continuing this line of investigation, we report in this communication partitioning studies in three water/organic biphase systems, as a function of PEO molar mass, aiming at factoring out the individual contributions of ethylene oxide and hydroxyl groups.